The invention is based on a method and an apparatus for producing a reaction mixture for forming solid and cellular substances from flowable reaction components and, optionally, fillers wherein the components and, optionally, fillers are premixed in a first mixing zone and subsequently mixed in a second mixing zone.
The production of such reaction mixtures immediately precedes the manufacture of molded articles in a mold and the continuous production of foam blocks or sandwich webs. The products formed are composed, for example, of polyurethanes (formed from a polyol and an isocyante), unsaturated polyester resins, epoxide resins, polyisocyanurate resins, or .epsilon.-caprolactam.
Mixing devices such as stirring mixers and injection mixers for the production of these reaction mixtures are known (U.S. Pat. No. 3,607,124).
Stirring mixers are used widely for mixing reaction components of low reactivity, with high viscosity differences and metering ratios. The stirring mixers known at present are characterized by a stationary sleeve which forms the outer contour of the mixing chamber and a stirrer which is arranged rotatably inside this sleeve (U.S. Pat. No. 3,607,124; U.S. Pat. No. 3,393,052). The seal between the stirrer shaft and the sleeve must be provided with a pressure-sensitive seal. When the stirrer is designed as a mixing screw having a pumping action from the components inlet to the mixing chamber outlet, this seal may, in some instances, be dispensed with. Due to the air gap which is inevitably present, however, undesirable air is sucked into the reaction mixture if the rate at which the components are metered is too low. Reaction mixture enters the air gap and solidifies, which leads to interruptions, if the metering rate is too high. These devices are suitable only for relatively low metering rates. In the production of substantially pore-free molded articles employing intermittent operation for filling molds, there are always problems if the mixing chamber has to be emptied and cleaned between two mold filling processes (i.e. between two mixing processes) due to the high reactivity of the reaction mixture present in the mixing chamber. As mixing begins again, the reactants enter the air-filled mixing chamber and small air bubbles are inevitably stirred into the reaction mixture, particularly when using spiked stirrers.
Another important feature of all stirring mixers is the reduction in the cross-section of the mixing chamber toward the outlet zone in which the reaction mixture leaves the mixing chamber through a hole (which is preferably round) or through a tube. It is also common to all stirring mixers that the intensity of mixing can be influenced during a mixing process only by changing the speed of the stirrer or the rate of metering because the geometry of the mixing chamber cannot be changed during the mixing process in most cases.
Self-cleaning injection mixers which have forced control do not usually have a stirrer and do not, therefore, have any sealing or cleaning problems (U.S. Pat. No. 3,263,928). They are suitable for processing highly active multi-component reaction systems even at high mixing chamber pressures and at delivery rates of up to about 600 l/min. Even when these mixers are provided with static aftermixers, it has been found that, with large viscosity differences in the reactants, with extreme metering ratios, with systems having high filler contents and with slight affinity of the reactants to each other, an adequate mixing effect is not guaranteed at the beginning or end of the mixing phase (U.S. Pat. No. 3,924,989).
The object of the instant invention is to provide a method and an apparatus wherein reactants having large viscosity differences and/or extreme metering ratios and/or with a high filler content and/or slight affinity of the reactants can be mixed perfectly throughout the entire mixing process. The apparatus is free from seals which are susceptible to failure and is operable when the mixing chamber is subjected to relatively high pressures.